Method of nitriding

ABSTRACT

A process for the nitriding of ferrous work utilizing a carrier gas consisting essentially of nitrogen with controlled low quantities of hydrogen. A ductile, wear resistant, hardened part is obtained by nitriding with a carrier gas containing 2-4 percent hydrogen, the balance being nitrogen, to which ammonia gas is added in quantities sufficient to prevent more than 10 percent hydrogen being present. Additionally, the ferrous parts are treated over a relatively short period of time, thereby obtaining a thin iron nitride layer on the surface of the work.

United States Patent [191 Lincoln et al.

1 July 1,1975

[ METHOD OF NITRIDING [75] Inventors: Joseph A. Lincoln; Joseph A.

Rlopelle, 111, both of Toledo. Ohio [73] Assignee: Midland-RossCorporation,

Cleveland, Ohio [22] Filed: Nov. 1, 1973 [21] Appl. No.: 411,850

Related U.S. Application Data [63] Continuation-impart of Ser. No.243,824, April I3.

1972, abandoned.

[52] U.S. Cl 148/16.6; 148/315 [51] Int. Cl. C23c 11/16 [58) Field ofSearch 148/121, 16.6, 31.5

[56] References Cited UNITED STATES PATENTS 2,452,9l5 ll/l948 Field148/166 3,399,085 8/1968 Knechtel 148/166 OTHER PUBLICATIONSTransactions of the Metallurgical Society of AIME,

Vol. 245, Jan. 1969, pgs. 161-163.

Zeitschrift fur Elektrochemie, Bd 36, No. 6, i930, pgs. 383-392.

Primary Examiner-C. Lovell Attorney, Agent, or Firm-Henry Kozak', Frank.I. Nawalanic ABSTRACT 1 Claim, 2 Drawing Figures METHOD OF NITRIDINGThis is a continuation-in-part of application Ser. No. 243,824, filedApr. 13, 1972 now abandoned.

BACKGROUND OF THE INVENTION Throughout the years, methods have beensought to improve the properties of ductility, hardness, high strengthand durability in ferrous parts. One means of achieving these propertiesis to nitride the parts to produce a nitrogen compound layer on thesurface of the ferrous parts and a nitrogen solution beneath the layer.Previous attempts have been made to nitride a ferrous workpiece througha method using a gaseous medium. These attempts have primarilyconcentrated on the cracking of ammonia to produce nascent nitrogenwhich will react with the metal to nitride the same. Mixtures of gasessuch as ammonia with hydrogen or endothermic gases have been suggestedin the past and high percentages of ammonia, at least 50 percent, alongwith hydrogen in excess of 20 percent had been used in these processes.These prior processes have had certain shortcomings, the basic one beingthat the gaseous nitriding processes resulted in brittleness beingimparted to the ferrous workpiece. Additionally, hydrogen gas isexpensive to produce and the high percentages of hydrogen create asafety hazard.

In order to overcome the shortcomings of the gaseous processes, variousinvestigators have turned to nitriding with a salt bath. This usuallyinvolves a form of cyanide and/or cyanate being used to nitride themetal. Although this has shown some success, the basic disadvantage isthat the cyanide process is an inconvenient and hazardous method ofaccomplishing the nitriding process. Contamination of the salt bath isanother problem associated with such a process. It obviously would beadvantageous to provide a process for nitriding ferrous parts through agaseous method which yields ductility, hardness and wear resistance.

It is therefore an object of this invention to provide a novel gaseousmethod of nitriding metal.

It is another object of this invention to nitride ferrous work using agaseous medium which results in the work having ductility, hardness andhigh fatigue strength.

It is still another object of nitriding ferrous parts in a process whichis safer to use than previous processes containing high quantities ofhydrogen.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 shows the effects on rate ofnitriding by maintaining the hydrogen constant and increasing theammonia.

FIG. 2 shows the effects on the rate of nitriding by maintaining theammonia constant and increasing the hydrogen.

SUMMARY OF THE INVENTION A process using a gaseous medium has been foundin which ferrous workpieces may be nitrided with the result thatdesirable properties are imparted to the workpieces. The gaseous mediumincludes a carrier gas consisting substantially of nitrogen butcontaining a relatively small amount of hydrogen into which isintroduced relatively small quantities of ammonia gas, these quantitiesof ammonia ranging from 5-25 percent. The amount of hydrogen present inthe carrier gas is preferably about 2-4 percent of the total carrier gasmixture. The process takes place at a temperature of approximatelyl,00()F. i 100 and the work is exposed to the gaseous atmosphere for arelatively short period of time, the average being approximately 4hours. In this way, a relatively thin compound layer of iron nitride isformed on the surface of the work and the nitrogen solution penetratessufficiently deep into the work to impart the desired property ofincreased fatigue strength.

DESCRIPTION OF THE PREFERRED EMBODIMENT The neutral or carrier gas inwhich the nitriding process takes place is first produced and introducedinto a suitable furnace. Such a carrier gas consisting substantially ofnitrogen with traces of CO and hydrogen may be produced exothermicallyby the reaction of air with methane or natural gas. The products ofcombustion are cooled, the CO is removed, as by a molecular sieve, andthe gas is dried to remove H O. The resulting carrier gas isapproximately -97percent nitrogen, we to l/% CO and 24% H and will bereferred to in the balance of this specification as carrier gas. It willbe appreciated that the carrier gas does not react in the nitridingprocess, but is the vehicle for exposing the workpieces to a desiredquantity or density of ammonia gas. As such, another neutral gas, suchas helium, may work equally as well as nitrogen in the carrier gas. Itshould be noted that the carrier gas is noncombustible because theamount of H is below the combustion level, which level is about 4percent. After the carrier gas has been introduced into the furnace, theferrous parts to be treated are placed in the furnace and are heated toa temperature of about l,00OF.

The parts are maintained at this temperature for a period of 1% to 10hours. This is done by first mixing a given quantity of ammonia withcarrier gas and then introducing the ammonia-carrier gas mixture intothe furnace so as to maintain a given percentage of ammonia in thefurnace. Next, ammonia is added to the carrier gas. The quantity ofammonia will vary depending upon the type of ferrous part being treated,the content vary ing from 5-25 percent of the furnace atmosphere. As theammonia is added to the heat treating chamber, it reacts with the hotferrous parts to form an iron nitride compound on the surface of theferrous parts. A sufficient quantity of the gas mixture within thefurnace is drawn off and new gas (ammonia-carrier gas mixture) is addedto control the composition of the treating gas so that the hydrogencontent in the furnace is at least 3 percent but does not exceed 10percent. Lower amounts of hydrogen are recommended because the equationFe xNH I, Fe .rN+ H is reversible and larger amounts of H cause thereaction to tend toward the left side of the equation thereby retardingthe nitriding reaction. In addition, an atmosphere containing only 3-10percent hydrogen is less hazardous, the combustion level beingapproximately 4% H thereby reducing the hazard of explosions.

EXAMPLE I Parts made of I035 steel were placed in a furnace in which thecarrier gas comprises 95-97% N and 24% H and heated to a temperature ofI,OSOF. Ammonia in the amount of l2 percent was added to the carriergas. The l035 steel was treated for approximately 4 hours after whichthe treatment was discontinued. An X-ray diffraction pattern wasconducted to determine the results achieved. It was found that a majorportion of the surface of the steel contained epsilon phase ironnitride, which is a solid solution of nitrogen and iron, and there wereno traces of Fe N or ferrite. A compound layer of 0.0005 inch thicknesswas found to have formed on the surface of the parts.

EXAMPLE ll Other parts made of l035 steel were heated to approximatelyl,050F. in a furnace containing the same carrier gas as in Example I,and into which ammonia in the amount of 17.2 percent was added. Thetreating was continued for 4 hours and the results by X-ray diffractionagain showed that the major portion of the surface of the steel wasepsilon phase iron nitride, there being no evidence of Fe,N or ferrite.

EXAMPLE III A sample of 4620 steel was nitrided in a furnace in whichthe carrier gas was the same as in Example I, and into which ammonia inthe amount of percent was added. Again the temperature of the heattreating was l,050F. and the time was for 4 hours. Once more it wasfound that the epsilon phase was evident and there were no traces of FeN or ferrite.

Physical testing of these examples showed that they had achieved asurface hardness of over Re 70 and that these samples were completelyductile. Thin shim stock which had been nitrided to produce a file hardsurface could be flexed 180 without fracture. All samples were found tohave a relatively thin, approximately 0.0005 inch, complex compoundlayer of various nitrides on the surface, and nitrogen in solution belowthis surface layer. The thin compound layer of complex nitrides givesincreased wear resistance to the sample, particularly when it is porousfree. The diffused region of nitrogen in solid solution will impartincreased fatigue strength.

Increasing the amount of ammonia in the atmosphere, while holding thehydrogen constant, caused the sample to not only gain more weight, butat an increased rate as shown in FIG. 1. On the other hand, increasingthe amount of hydrogen while holding the am monia constant caused amarked decrease in the weight gain as shown in FIG. 2. These effects canbe illustrated using the ammonia breakdown equation:

ZNH (gas) --u 2E (in iron) 3H (gas) Ammonia in equilibrium with nitrogenin iron plus hydrogen gas.

As stated previously, increasing the ammonia would tend to shift theequation to the right, to form more nitrogen in iron; whereas,increasing the hydrogen would tend to shift the equation to the left, toform less nitro gen in iron, which is what was noted in weight gaintests using a recording balance. The magnitude and rate of these changeswere demonstrated by these tests.

Determination of the effects of ammonia and hydrogen on the thicknessand composition of the compound layer was made using microscopictechniques and X-ray diffraction techniques. As would be expected,higher ammonia contents produced a thicker compound layer; however, theporosity and brittleness were also increased. Using an atmosphere havingbetween 15 percent and 25 percent ammonia, the compound layer wasrelatively pore-free. Increasing the hydrogen content at a constantammonia decreased the compound layer thickness but its effect was notnearly as pronounced as noted with variations in ammonia. This effectwas demonstrated by a test on shim stock of 1008 steel.

In the first run, the 1008 steel shim stock sample was heated to atemperature of 1,050F. within a furnace containing a gas mixture of 92percent NH;, and 8% H The treatment was maintained for 4 hours. Thecompound layer was examined and it was found to have 40 percentporosity.

A second run was made with similar 1008 steel samples under the sameconditions as above with the exception that the gas mixture within thefurnace contained 25.5% Nl-I and 4.9% H the balance being nitrogen. Thesurface layer was found to have 15 percent porosity, which figure is atthe threshold of acceptability.

One final run was made under the same conditions with the exception thatthe gas mixture within the furnace was 15% Nl-l and 5.6% H the balancebeing nitrogen. No porosity was detected in these samples.

Another important observation was noted from these samples. The overallthickness of the samples was approximately 0.008 inch and this thicknessdid not appreciably change even with the high nitrogen compound layersamples. This indicates that the compound layer is not built up on thesurface, but instead just alters the surface layer of iron to form ironnitride. Some growth will probably take place due to the addition ofnitrogen into the steel, but its magnitude is not large enough to allowthe change in thickness to be measured. This is important for at leasttwo reasons: (1) Materials which are not homogeneous, such as castirons, and have graphite flakes, or nodules, that extend all the way tothe surface, will not form a complete compound layer, but will havefatigue strength increased even with the presence of stress risers. (2)Dimensional changes and distortions with nitrided parts are held to aminimum.

What is claimed is:

l. A process for nitriding ferrous parts to produce a thin compoundlayer of complex nitrides containing principally epsilon phase withoutthe presence of Fe N comprising the steps of:

placing said parts in a furnace containing an atmosphere defined as anoncombustible carrier gas consisting essentially of95-97 percentnitrogen, /2 to 1 /2 percent carbon monoxide and 2-4 percent hydrogen;

heating said parts to an approximate temperature of introducing ammoniagas to said carrier gas to form a nitriding atmosphere within thefurnace, said nitriding atmosphere containing 525 percent ammonia gas byvolume;

maintaining the hydrogen gas in said nitriding atmo' sphere at a valueno greater than 10 percent by voltune in said nitriding atmosphere byintroducing additional quantities of carrier gas while withdrawingportions of said nitriding atmosphere; and subjecting said parts to saidnitriding atmosphere for a period between /2 to l0 hours until acompound layer of approximately 0.0005 inch thickness and composedprincipally of epsilon phase exists at the surface of said parts.

* 3F k i

1. A PROCESS FOR NITRIDING FERROUS PARTS TO PRODUCE A THIN COMPOUNDLAYER OF COMPLEX NITRIDES CONTAINING PRINCIPALLY EPSILON PHASE WITHOUTTHE PRESENCE OF FE4N COMPRISING THE STEPS OF: PLACING SAID PARTS IN AFURNACE CONTAINING AN ATMOSPHERE DEFINED AS A NONCOMBUSTIBLE CARRIER GASCONSISTING ESSENTIALLY OF 95-97 PERCENT NITROGEN, 1/2 TO 1 1/2 PERCENTCARBON MONOXIDE AND 2-4 PERCENT HYDROGEN, HEATING SAID PARTS TO ANAPPROXIMATE TEMPERATURE 1,000*F, INTRODUCING AMMONIA GAS TO SAID CARRIERGAS TO FORM A NITRIDING ATMOSPHERE WITHIN THE FURNACE , SAID NITRIDINGATMOSPHERE CONTAINING 5-25 PERCENT AMMONIA GAS BY VOLUME, MAINTAININGTHE HYDROGEN GAS IN SAID NITRIDING ATMOSPHERE AT A VALUE NO GREATER THAN10 PERCENT BY VOLUME IN SAID NITRIDING ATMOSPHRE BY INTRODUCINGADDITIONAL QUANTITIES OF CARRIER GAS WHILE WITHDRAWING PORTIONS OF SAIDNITRIDING ATMOSPHERE, AND SUBJECTING SAID PARTS TO SAID NITRIDINGATMOSPHERE FOR A PERIOD BETWEEN 1/2 TO 10 HOURS UNTIL A COMPOUND LAYEROF APPROXIMATELY 0.0005 INCH THICKNESS AND COMPOSED PRINCIPALLY OFEPSILON PHASE EXISTS AT THE SURFACE OF SAID PARTS.